[Markdown] Format our returns as heading to match the params.

[10Derozan]

If you're reporting a bug, please include input code, output documentation,
a description of what you expected to happen, and what happened instead.

When the output is markdown, don't you think we should also format our returns as heading to match the params?
https://github.com/documentationjs/documentation/blob/master/src/output/markdown_ast.js#L181
https://github.com/documentationjs/documentation/blob/master/src/output/markdown_ast.js#L111

• What version of documentation.js are you using?: 14.0.2
• How are you running documentation.js (on the CLI, Node.js API, Grunt, other?):
  Node.js API